U.S. patent number 5,975,462 [Application Number 09/140,970] was granted by the patent office on 1999-11-02 for integrated propulsion/lift/control system for aircraft and ship applications.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Navy. Invention is credited to Maximilian F. Platzer.
United States Patent |
5,975,462 |
Platzer |
November 2, 1999 |
Integrated propulsion/lift/control system for aircraft and ship
applications
Abstract
A new method for boundary layer energization and boundary layer
propulsion or use on vehicles moving through fluids, which
comprises mounting small airfoils parallel or perpendicular to the
vehicle's surface, said airfoils being embedded within the said
vehicle's boundary layer and juxtaposed the surface of said
vehicle, said airfoils being approximately the height of the
boundary layer thickness and exciting said airfoils into flapping
oscillation parallel to the chord plane of said airfoils, said
oscillation at a frequency up to 100 cycles per second at an
amplitude up to 20 percent of the chord length of said airfoil,
whereby flow separation is delayed or suppressed which enables the
redesign of said vehicle.
Inventors: |
Platzer; Maximilian F. (Pebble
Beach, CA) |
Assignee: |
The United States of America as
represented by the Secretary of the Navy (Washington,
DC)
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Family
ID: |
24973674 |
Appl.
No.: |
09/140,970 |
Filed: |
August 27, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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739761 |
Oct 30, 1996 |
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Current U.S.
Class: |
244/11; 244/130;
244/72; 244/204 |
Current CPC
Class: |
B64C
19/02 (20130101); B63H 1/30 (20130101); B63H
1/28 (20130101); B64C 21/00 (20130101); B64C
33/00 (20130101); B63B 1/34 (20130101); Y02T
70/10 (20130101); Y02T 50/10 (20130101) |
Current International
Class: |
B64C
19/00 (20060101); B64C 19/02 (20060101); B63H
1/28 (20060101); B63H 1/30 (20060101); B63H
1/00 (20060101); B63B 1/24 (20060101); B63B
1/00 (20060101); B64C 21/00 (20060101); B63B
1/16 (20060101); B63B 1/34 (20060101); B64C
33/00 (20060101); B64C 021/00 () |
Field of
Search: |
;244/204,203,201,198,130,72,22,11,215,213 ;440/13 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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586496 |
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Oct 1933 |
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DE |
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2107426 |
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Apr 1983 |
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GB |
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Primary Examiner: Barefoot; Galen L.
Attorney, Agent or Firm: Lincoln; Donald E.
Parent Case Text
This is a continuation of earlier filed application Ser. No.
08/739,761 of Maximilian F. Platzer, filed on Oct. 30, 1996 now
abandoned.
Claims
What is claimed is:
1. A new method for boundary layer energization and boundary layer
propulsion for use on vehicles moving through fluids, which
comprises:
mounting at least one small airfoil parallel to said vehicle's
surface, said airfoil being embedded within said vehicle's boundary
layer; and
exciting said airfoil into flapping oscillation parallel to the
chord plane of said airfoil at a frequency up to 100 cycles per
second at an amplitude up to 20 percent of the chord length of said
airfoil for generating a jet flow downstream of the airfoil as a
direct result of its oscillation for the purpose of re-energizing
and accelerating the boundary layer and thereby preventing flow
separation and wake formation, whereby flow separation is delayed
or suppressed on said vehicle which enables the re-design of the
shape of said vehicle.
2. The method of claim 1, wherein:
the vehicle is an air vehicle.
3. The method of claim 1, wherein:
the vehicle is a marine vehicle.
4. A new method for energization and suppression of wakes shed from
vehicles moving through fluids, which comprises:
mounting at least one airfoil in said vehicle's wake; and
exciting said airfoils into a flapping oscillation parallel to the
chord plane of said airfoil, with a frequency up to 100 cycles per
second at an amplitude up to 20 percent of the chord length of said
airfoil for generating a jet flow downstream of the airfoil as a
direct result of its oscillation for the purpose of re-energizing
and accelerating the boundary layer and thereby preventing flow
separation and wake formation.
5. The method of claim 4, wherein:
the vehicle is an air vehicle.
6. The method of claim 4, wherein:
the vehicle is a marine vehicle.
7. A new method for energization and suppression of wakes shed from
vehicles moving through fluids, which comprises:
mounting a pair of airfoils in said vehicle's wake; and
exciting said airfoils into a flapping oscillation whereby said
airfoils are flapping in counterphase, said oscillation occurring
parallel to the chord plane of said airfoils at a frequency up to
100 cycles per second at an amplitude up to 20 percent of the chord
length of said airfoils for generating a jet flow downstream of the
airfoil as a direct result of its oscillation for the purpose of
re-eneroizing and accelerating the boundary layer and thereby
preventing flow separation and wake formation.
8. The method of claim 7, wherein:
the vehicle is an air vehicle.
9. The method of claim 7, wherein:
the vehicle is a marine vehicle.
10. A new method for boundary layer energization of fluids moving
through a structure, which comprises:
mounting at least one small airfoil proximate the fluid boundary
and juxtaposed the surface of said structure; and
exciting said airfoils into a flapping oscillation at a frequency
up to 100 cycles per second at an amplitude up to 20 percent of the
chord length of said airfoils for generating a jet flow downstream
of the airfoil as a direct result of its oscillation for the
purpose of re-energizing and accelerating the boundary layer and
thereby preventing flow separation and wake formation, whereby flow
separation is delayed or suppressed which enables the
reconfiguration of the surface of said structure.
11. The method of claim 10, wherein:
the vehicle is a flow ducdiffuser.
12. A new method for boundary layer energization and boundary layer
propulsion for use on vehicles moving through fluids, which
comprises:
mounting small airfoils perpendicular to the vehicle's surface,
said airfoils being embedded within the said vehicle's boundary
layer and juxtaposed the surface of said vehicle, said airfoils
being approximately the height of the boundary layer thickness;
and
exciting said airfoils into flapping oscillation parallel to the
chord plane of said airfoils, said oscillation at a frequency up to
100 cycles per second at an amplitude up to 20 percent of the chord
length of said airfoil for generating a jet flow downstream of the
airfoil as a direct result of its oscillation for the purpose of
re-energizing and accelerating the boundary layer and thereby
preventing flow separation and wake formation, whereby flow
separation is delayed or suppressed which enables the
reconfiguration of said vehicle.
13. The method of claim 12, wherein:
the vehicle is an air vehicle.
14. The method of claim 12, wherein:
the vehicle is a marine vehicle.
15. A new method of generating thrust for use on vertical/short
take-off and landing (V/STOL) aircraft, which comprises:
mounting at least one airfoil proximate to and parallel to said
aircraft's fuselage; and
exciting said airfoil into a flapping oscillation parallel to the
chord plane of said airfoil, said oscillation at a frequency up to
100 cycles per second at an amplitude up to 20 percent of said
chord length of said airfoil for generating a jet flow downstream
of the airfoil as a direct result of its oscillation for the
purpose of re-energizing and accelerating the boundary layer and
thereby preventing flow separation and wake formation.
16. A new method of generating thrust for use on vertical/short
take-off and landing (V/STOL) aircraft, which comprises:
mounting at least one pair of airfoils proximate to and parallel to
said aircraft's fuselage; and
exciting said pair of airfoils into a counter-phase type of
flapping oscillation parallel to the chord plane of said airfoils,
said oscillation at a frequency up to 100 cycles per second at an
amplitude up to 20 percent of the chord length of said airfoils for
generating a jet flow downstream of the airfoil as a direct result
of its oscillation for the purpose of re-energizing and
accelerating the boundary layer and thereby preventing flow
separation and wake formation, whereby ajet flow is generated by
said flapping airfoils which is directed into a thrust augmenting
ejector.
Description
BACKGROUND OF THE INVENTION
1. Filed of the Invention
This invention relates to aircraft and ship boundary layer
propulsion (thrust generation) mechanisms employing flapping foils.
It further relates to aerodynamic or hydrodynamic lift systems
employing flapping foils. This invention also relates to
aerodynamic and hydrodynamic drag generated on aircraft and ships.
More precisely, it relates to aerodynamic/hydrodynamic control
systems employing flapping foils, including the delay or prevention
of flow separation in general fluid flow systems.
2. Background of the Invention
It is firmly established that fluid flow over most air and marine
vehicles occurs at such large Reynolds numbers that the viscous
forces acting on the fluid particles are of importance only in a
very thin layer surrounding the vehicle. The remainder of the fluid
flow field can be regarded as inviscid (frictionless). The Reynolds
number signifies the ratio of the fluid particle's inertia force to
the viscous force. Typically, the inertia force is several hundred
thousand or million times larger than the viscous force. As a
result, the thin fluid particle layer (which is usually referred to
as "boundary layer") developing on an aircraft wing typically has a
thickness of only one or two percent of the aircraft wing's chord
length.
Over the years, it has also been recognized that the type of fluid
flow which develops in the boundary layer has a decisive effect on
the air or marine vehicle's drag. One distinguishes between the two
types of fluid flow, namely "laminar flow" and "turbulent flow".
The laminar fluid flow is a well-ordered steady flow, whereas the
turbulent fluid flow is characterized by small irregular
fluctuations about a steady mean flow. The friction force exerted
by a laminar flow's boundary layer on the vehicle's outer surface
exposed to the laminar flow is significantly smaller than the
friction force generated by a turbulent flow's boundary layer. On
the other hand, a laminar flow boundary layer has a much greater
proclivity to separate (detach) from the vehicle's surface than a
turbulent flow boundary layer. Separated flow produces a different
type of drag, which is usually referred to as "pressure drag".
Large regions of flow separation may produce huge increases in
pressure drag. Hence efficient air or marine vehicle design
requires the minimization of both skin friction and pressure
drag.
DESCRIPTION OF THE RELATED ART
Heretofore, the problems of (i) thrust generation and (ii) drag
reduction were generally regarded as two separate independent
problems for most vehicles with the exception of hypersonic and
aerospace planes. There is one notable exception, however. F. H.
Reynst proposed, in the book "Pulsating Combustion", Pergamon Press
1961, the boundary layer propulsion concept and the use of
pulsating combustors for its implementation. An analysis of the
advantages of boundary layer propulsion was also published by A. M.
O. Smith and H. E. Howard in the Journal of Aeronautical Sciences,
February 1947, pp. 97-109. Unfortunately, no work was done on
Reynst's proposed boundary layer propulsion system and therefore no
information exists on the practicality and efficiency of his
propulsion system.
It is clear from the above description of the boundary layer
behavior that great benefits in terms of lift increase and/or drag
reduction can be achieved by effective boundary layer control. For
this reason, much effort has been expended over the years to devise
and demonstrate various boundary layer control methods. These can
be classified as follows:
a) "Boundary Layer Suction." As reviewed, for example, in the two
volumes on "Boundary Layer and Flow Control", edited by G. V.
Lachmann' Pergamon Press 1961, the most obvious boundary layer
control method is the removal of most of the boundary layer over
most of the air or marine vehicle by means of a suitable suction
system. The importance of increasing the stealthiness of a surface
ship has recently been pointed out by R. J. Furey in his U.S. Pat.
No. 5,222,455 dated Jun. 29, 1993 (assigned to the U.S. Navy). He
proposed to reduce or eliminate a ship's wake by sucking off the
portion of the boundary layer closest to the hull and expelling it
alongside the ship.
b) "Blowing." Again, as reviewed by Lachmann, another way of
influencing (and energizing) the boundary layer is by means of air
blown from certain strategic locations on the vehicle. For example,
an air jet blown from the wing's trailing edge has a significant
effect on lift and drag. Such systems are known by the name "jet
flap" and "circulation control" systems. They have been
demonstrated to be quite effective for certain purposes. However,
the ducting required for channeling the air from the jet engine to
the trailing edge and the losses inherent in such systems are major
obstacles to their adoption.
c) "Lift enhancement." The problem of suppressing flow separation
becomes particularly acute in the design of high-lift airfoils. As
shown by Liebeck in the Journal of Aircraft, Vol. 7, pp. 409-415,
September-October 1970, the process of flow deceleration from the
suction peak near the leading edge to the trailing edge on the
airfoils upper surface is governed by the maximum allowable adverse
pressure gradient.
d) "Boundary Layer Tripping Devices." Still another method of
boundary layer control is to trigger the transition of the laminar
boundary layer into a turbulent boundary layer by means of small
protuberances on the airfoil surface.
e) "Riblets and Large Eddy Breakup Devices." In recent years it has
also been recognized that small-geometry surface configurations can
have a beneficial (i.e., drag reduction) effect on the turbulent
boundary layer. U.S. Pat. No. 5,386,955, issued on Feb. 7, 1995,
awarded to M. Savill of Rolls Royce, falls into this category,
wherein a series of streamwise riblets of different heights is
arranged.
f) "Boundary Layer Propulsion." The idea of boundary layer
propulsion seems to have been first proposed and analyzed by A. M.
O. Smith and H. E. Howard in their paper "The Jet Airplane
Utilizing Boundary Layer Air for Propulsion", J. Aeronautical
Sciences, February 1947, pp. 97-109 and by F. H. Reynst in a series
of papers published in the book "Pulsating Combustion", Pergamon
Press 1961. Both studies showed significant benefits derivable from
the use of the boundary layer air instead of the free-stream air
(as is done in conventional systems). Reynst had also developed a
new pulsating combustion engine which had no moving mechanical
parts. This engine used pulsating combustion similar to the
pulse-jet engine developed in Germany during World War II and used
on the V-1 flying bombs. In contrast to the V-1 engine, however, no
mechanical valves were required. Because of the simplicity of his
engine, Reynst felt that the arrangement of pulsating combustion
engines along the wing trailing edge could be a practical boundary
layer propulsion system. No such system was ever built, however,
and it is questionable whether the inherently low propulsive
efficiencies of pulsating combustion engines would ever lead to a
practical system. Ideally, the slowing down of the fluid particles
in the boundary layer and the resulting momentum loss (and
therefore drag production) should be prevented by the installation
of small propellers along the wing's trailing edge. In this way,
the development of an adverse pressure gradient region could be
avoided and the transition from laminar to turbulent flow and the
formation of a separated flow region near the trailing edge could
be prevented. Unfortunately, the use of small propellers would pose
an extremely complicated mechanical installation problem.
Potentially, the propellers could be replaced by small jet engines.
However, here again the development and installation of small
engines would be quite difficult.
g) "Flapping Foil Propulsion." Thrust generation due to wing
flapping was first explained by Knoller in Flug-und Motortechnik,
Wien, Vol. 3, No. 21, pp. 1-7, 1909 and by Betz in Zeitschrift fur
Flugtechnik und Motorluftschiffahrt, Vol. 3, pp. 269-272, January
1912. In 1936, Garrick showed in NACA Report 567 that pure wing
flapping (plunging without pitching) produces thrust at a
propulsive efficiency of only 50%, unless the wing oscillation
occurs at very low frequency. The interest in using flapping foil
propulsors therefore remained low in subsequent years. However, in
1965 W. Schmidt showed in Zeitschrift fur Flugwissenschaften, Vol.
13, pp. 472-479 that the propulsive efficiency can be increased
quite substantially if a tandem airfoil combination is used whereby
the forward airfoil is excited into a flapping motion while the
rearward airfoil is kept stationary. He called this propulsion
system a wave propeller and he demonstrated its applicability on a
catamaran boat. It appears that this German work remained unknown
in the United States because in 1968 Scherer, of Hydronautics
Research Inc., proposed a flapping foil propulsor for use on a
shallow-draft boat which used only single flapping airfoils. Quite
recently, Triantafyllou and Barrett recognized that the relatively
low propulsive efficiency of the single-foil pure-flapping
propulsor can be improved by combining the flapping motion with a
simultaneous pitching motion. This system is described in their
U.S. Pat. No. 5,401,196, issued Mar. 28, 1995.
SUMMARY OF THE INVENTION
It is argued that the concept of boundary layer propulsion is
sound, but that it is critically dependent on the use of an
efficient and practical propulsion system which is easy to install
and operate, and which makes it possible to capture and accelerate
the boundary layer flow over an air or marine vehicle. A need
therefore exists for a propulsion mechanism suitable for this task.
From the discussion of the Related Art, above, it can be seen that
not only recognizing the importance of capturing the boundary
layer, but also accelerating it as a method for increasing
propulsion and lift while also reducing drag has escaped those
skilled in the art until now. The implementation of the boundary
layer propulsion concept, therefore, is critically dependent on the
use of an efficient and practical propulsion (thrust) system which
is easy to install and operate and which makes it possible to
capture and accelerate the boundary layer flow.
Accordingly, this invention addresses the problem of drag reduction
of air or marine vehicles. The efficient and practical boundary
layer energization offered by this invention makes it possible to
design airfoils and ship hulls which permit the delay of boundary
layer transition from laminar to turbulent flow. Laminar flow
profiles have a substantially lower skin friction and hence lower
overall drag. However, such profiles are also prone to boundary
layer separation unless active boundary layer control is used. As
outlined above, until now active boundary layer control was limited
to boundary layer suction or blowing schemes. The proposed
invention offers a third means of active boundary layer
control.
The invention also addresses the problem of designing airfoils with
significantly increased adverse pressure gradient regions without
incurring flow separation. Such airfoils will generate higher lift
coefficients.
This invention also addresses the problem of control force
generation by permitting a simple and fast action method of
generating control forces by the use of a three-element airfoil
configuration. Rotation (angle of attack changes) of the
conventional flap downstream of the flapping foil will change the
lift generated on the flap and hence will produce significant
control moment increments.
This invention can also be applied to minimize or to prevent the
formation of wakes behind airfoils and wings.
The invention can also be applied to any duct flow problem where
flow separation may be encountered due to an adverse pressure
gradient. Flapping foil flow control can be used to suppress flow
separation.
OBJECTS OF THIS INVENTION
It is an object of this invention to provide such a system which is
based on the addition of crucial new elements to the already known
flapping foil propulsion systems.
It is a further object of this invention to provide a propulsion
mechanism for use on aircraft wings and fuselages and on ship hulls
such that the boundary layer is captured, is prevented from flow
separation, and is given an additional momentum. In this way the
pressure drag is minimized and additional thrust is generated. To
achieve this objective, one or more small flapping foils are
mounted close to the wing of an aircraft near the wing's trailing
edge or close to the hull surface on a ship's stern. Although
Schmidt and Triantafyllou & Barrett have presented flapping
foil propulsors with higher propulsive efficiencies than previously
attainable with single-foil pure-flapping propulsors, they have
failed to recognize the crucial importance of boundary layer
propulsion, namely (i) to prevent the formation of a significant
boundary layer along the wetted surface, (ii) to suppress the
formation of a significant wake immediately downstream of the
airfoils trailing edge or the ship's stern, and, most importantly,
(iii) to suppress boundary layer flow separation on the airfoil or
ship hull.
Another object of this invention is to recognize that efficient
boundary layer propulsion can be achieved by arranging flapping
airfoils in the boundary layer for the purpose of re-energizing and
accelerating the boundary layer and thereby preventing flow
separation and wake formation.
It is a further object of this invention to provide a propulsion
mechanism for use on aircraft wings and fuselages, and on ship
hulls such that the boundary layer is captured, is prevented from
flow separation, and is given an additional momentum. In this way
the pressure drag is minimized and additional thrust is generated.
To achieve this objective, one or more small foils are mounted
close to the wing of an aircraft near the wing's trailing edge or
close to the hull surface on a ship's stern. These foils are
excited into a pure flapping (heaving or plunging) motion by means
of a suitable mechanical, electromechanical or hydraulic excitation
system. The flapping motion, typically, will be purely sinusoidal
with the maximum attainable frequency. The flapping amplitude,
typically, will be a fraction of the foil's chord length, say 5 or
10 percent of chord. A crucial element of the proposed invention is
the wing or hull shape in the vicinity of the flapping foils.
These objects and other objects and advantages of the present
invention will become more fully understood by reference to the
following Description taken in conjunction with the attached
Drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
A more complete appreciation of the invention will be readily
obtained by reference to the following Description of the Preferred
Embodiments and the accompanying drawings in which like numerals in
different figures represent the same structures or elements. The
representation in each of the figures is diagrammatic and no
attempt is made to indicate actual scales or precise ratios.
FIG. 1, 1a, 1b, & 1c depict an airfoil showing air flowing over
it to illustrate the various fluid flow features (laminar,
transitional, turbulent boundary layer flow and flow
separation).
FIG. 2 depicts an airfoil showing relationship between the lower
surface of the foil having larger pressure than the upper surface,
thereby creating lift on the airfoil.
FIG. 3 depicts an airfoil with air passing over it and the reduced
velocity of air immediately behind the airfoil. This situation
produces drag.
FIG. 4 depicts an airfoil energized into a flapping motion which
creates a higher (i.e., a jet) velocity in the air directly behind
the airfoil. The jet indicates the generation of thrust.
FIG. 5 depicts the thrust obtained by a flapping airfoil, whereby
the thrust increases as the reduced frequency is increased.
FIG. 6 is a graph obtained in the Naval Postgraduate School (NPS)
water tunnel tests, showing the increase in jet velocity and
therefore increase in thrust as the foil flaps juxtapose a flat
plate. The foil is located in the boundary layer if spacing y/delta
is less than one. (Note: y is the distance of the foil from the
plate and U is the boundary layer thickness).
FIG. 7 depicts an airfoil showing two small flapping airfoils
placed in the adverse pressure gradient area of the top surface of
the wing near the trailing edge.
FIG. 8 depicts two different airfoil tested in the NPS water
tunnel, one with a cusped trailing edge and the other a round
trailing edge.
FIG. 9 depicts the graph obtained from the NPS water tunnel tests
to confirm amplitude required for re-attachment of fluid flow to a
ship's hull.
FIG. 10 depicts the top view of the aircraft wing shown in FIG. 7.
It is seen that the flapping airfoils are mounted parallel to the
wing surface.
FIG. 11 depicts a ship hull with a cusped trailing edge and
flapping foils both attached to the hull and trailing behind the
hull.
FIG. 12 depicts an aircraft wing with multiple small flapping
airfoils mounted on the top and perpendicular to the wing while
flapping in the spanwise direction, i.e., perpendicular to the
chord plane of the airfoils.
FIG. 12a depicts a cross sectional view of FIG. 12.
FIG. 13 depicts a square duct with the flow of a fluid flowing from
left to right where the flow passes over flapping foils placed
close to the duct walls.
FIG. 13a depicts a cross sectional view of FIG. 13.
FIG. 14 depicts a thrust augmenting ejector where the primary jet
flow is induced by the flapping airfoil.
While the present invention is susceptible to various modifications
and alternative forms, specific embodiments have been shown by way
of example in the Drawings and will be described in detail herein.
It should be understood, however, that they are not intended to
limit the invention to the particular forms disclosed. On the
contrary, the invention is to cover all modifications, equivalents,
and alternatives falling within the spirit and scope of the
invention.
GLOSSARY
The following list of elements are referred to in the drawings
representing a New Integrated Propulsion--Lift--Control System for
Aircraft and Ship Applications.
5. AIRFOIL
6. AIRFOIL CHORD
7. AIRFOIL UPPER SURFACE
8. AIRFOIL LOWER SURFACE
10. BOUNDARY LAYER [the thin layer of air in contact/adjacent to a
solid surface over which air or water is flowing. The Boundary
Layer begins at the surface of the aircraft where the air in
immediate contact with the surface is at zero velocity relative to
the surface; the outer edge of the Boundary Layer is at air-stream
velocity. There is thus a velocity gradient in this layer and a
stress is produced in the layer. The skin-friction drag is the
result of the stresses produced in this layer. The Boundary Layer
starts as a "Laminar Layer which terminates abruptly in a
Transition Region after which the Boundary Layer becomes Turbulent
and increases in thickness. The Boundary Layer is illustrated in
FIG. 1.
It is known that the drag of the Laminar Boundary Layer is much
lower than the drag of the Turbulent Layer, hence it is important
to design for Laminar Layers! The Transition from the Laminar
Boundary Layer Flow to the Turbulent Layer occurs at a critical
value of the Reynolds number.
12. FRONT STAGNATION POINT
15. LAMINAR BOUNDARY LAYER
20. WETTED SURFACE
25. TURBULENT LAYER
30. REYNOLDS NUMBER=Inertial Force/Viscous Force
35. SEPARATED FLOW (Flow Separation)
40. PRESSURE DRAG
45. FLUID PARTICLE
50. REAR STAGNATION POINT
55. PRESSURE DROP AREA
60. PRESSURE DIFFERENCE
65. STATIC PRESSURE of the undisturbed flow
70. SKIN FRICTION
75. BOUNDARY LAYER SEPARATION
76. DUCT
77. DUCT FLOW
80. SEPARATED FLOW REGION
85. TURBULENT BOUNDARY LAYER
90. LAMINAR BOUNDARY LAYER
95. MOMENTUM LOSS
100. ADVERSE PRESSURE REGION
105. FREE-STREAM AIR
110. THRUST
115. FLAPPING FOIL PROPULSORS
120. WAVE PROPELLER
125. WAKE (Wake formation)
130. BOUNDARY LAYER REDUCTION
135. RE-ENERGIZING AND ACCELERATING the boundary layer
140. FLAPPING AIRFOIL
145. ##EQU1## 150. INCREASING REDUCED FREQUENCY 155. JET EFFECT/JET
VELOCITY
160. MOMENTUM INCREASE
165. THRUST DEPENDENCE
170. PUMPING EFFECT OF A FLAPPING FOIL
175. VELOCITY DISTRIBUTION
180. SMALL AIRFOIL oscillated in the flapping mode
185. WALL EFFECT
190. DRAG/PRESSURE DRAG
195. BOUNDARY LAYER PROPULSION
200. BOUNDARY LAYER TRANSITION from Laminar Flow to Turbulent
Flow
205. LIFT/HIGH LIFT/HIGHER-LIFT COEFFICIENTS
210. SUCTION PEAK
213. FAVORABLE PRESSURE GRADIENT--BOUNDARY LAYER flow proceeding
along the airfoil surface from a region of high pressure to one of
low pressure.
215. ADVERSE PRESSURE GRADIENT--BOUNDARY LAYER flow proceeding
along the airfoil surface from a region of low pressure to one of
high pressure, the boundary layer flow will almost immediately
undergo TRANSITION FROM THE LAMINAR STATE TO THE TURBULENT
STATE.
216. BOUNDARY LAYER PHENOMENON--There is a tendency of the boundary
layer to become thicker and thicker as the flow proceeds against an
adverse pressure gradient. The velocity U of the layers of air next
to the surface becomes more and more retarded until the velocity
above the surface, instead of increasing, remains equal to zero for
a short distance. Under these conditions, the air will react to the
higher pressure downstream, and will actually begin to flow
upstream along the surface, forcing the free stream flow away from
the airfoil surface. Such a flow is said to be "Separated", and the
point at which it occurs is termed the "Separation Point".
220. CONTROL FORCES
225. CONTROL MOMENT/CONTROL MOMENT INCREMENTS
230. JET-LIKE VELOCITY PROFILE
235. BOUNDARY LAYER PROPULSION
240. PULSATING COMBUSTORS
245. FAVORABLE PRESSURE GRADIENT
250. RECOMPRESSION
255. LOW PRESSURE ZONE
260. FREE-STREAM PRESSURE
265. FLAPPING FOIL PROPULSORS
270. CUSPED SHAPE
275. ADVERSE PRESSURE GRADIENT REGION
280. INDUCED DRAG=drag due to the production of lift
285. TOTAL RESISTANCE of an aircraft is composed of three parts:
SKIN FRICTION DRAG due to viscosity of air; form or PRESSURE DRAG,
and the drag due to production of LIFT (INDUCED DRAG).
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
It is important to recognize that the boundary layer 10 grows from
zero thickness at the front stagnation point 12 of an airfoil 5 to
significant values near the rear stagnation point 50, as
illustrated in FIG. 1. If an airfoil shape is to be found which
produces high lift 205 and small drag 190, the speed of the fluid
particles 45 which flow near the airfoil's upper surface 7 (see
FIG. 2) needs to be increased to large values so that (according to
Bernoulli's law) a large pressure drop 55 can be achieved. This, in
turn, will produce a significant pressure difference 60 between the
lower 8 (see FIG. 2) and upper 7 airfoil surface and hence produce
a significant lifting force 207. However, near the trailing edge of
the airfoil 122 the pressure 251 has to approach the static
pressure 252 of the undisturbed air flow 253. This requires the
fluid particles 45 near the upper surface 7 to flow into a region
of increased pressure (adverse pressure gradient region 100). As a
consequence, the flow in the boundary layer 10 quickly changes from
laminar flow 23 to turbulent flow 24 and hence increases the skin
friction or, even worse, the boundary layer 10 starts to separate
75 and a large pressure drag 40 is produced. Even when the boundary
layer 10 stays attached over most of the airfoil 5, its thickness
may increase to ten or fifteen percent of airfoil chord 6 and a
small separated flow region 35 always develops near the trailing
edge 122.
The crucial new element introduced by the present invention is the
recognition that efficient boundary layer propulsion 195 can be
achieved by arranging flapping (plunging) airfoils 140 in the
boundary layer 10 for the purpose of re-energizing and accelerating
the boundary layer 10 and thereby preventing flow separation 35 and
wake formation 125. The air flow behind the stationary
(non-flapping) airfoil 5 typically has the distribution shown in
FIG. 3. In fact, the reduction in velocity (called the wake 125) is
a direct measure of the drag 39 generated by the airfoil S. In
contrast, FIG. 4 shows the measured and computed velocity
distribution behind a flapping [plunging] airfoil 140. It is seen
that flapping [plunging] of an airfoil 140 generates a significant
velocity increase (Jet Effect 155 ) over the free stream air speed
105 (by almost a factor of two). Hence airfoil flapping [plunging]
produces a jet effect 155 which is a direct measure of the thrust
110 (rather than drag 190)) produced by the flapping [plunging]
airfoil 140. This jet effect 155 is significantly enhanced if the
airfoil is flapping near a flat plate or near an airfoil surface
(wall effect 185). These measurements and computations were
performed at NPS and are documented by I. H. Tuncer and M. F.
Platzer, "Thrust Generation due to Airfoil Flapping, AIAA Journal,
Vol. 34, No. 2, pp. 324-331, February 1996; by K. D. Jones, C. M.
Dohring, M. F. Platzer, "Wake Structures behind Plunging Airfoils:
A Comparison of Numerical and Experimental Results," AIAA Paper No.
96-0078, January 1996; and by C. M. Dohring, M. F. Platzer, K. D.
Jones, I. H. Tuncer, "Computational and Experimental Investigation
of the Wakes shed from Flapping Airfoils and their Wake
Interference/impingement Characteristics," 78th AGARD Fluid
Dynamics Panel Symposium, Paper No. 33, Trondheim, Norway, 20-23
May 1996.
It is important to note two effects which have not been recognized
in the past and which demonstrate the superiority and practicality
of the new boundary layer propulsion system over previous systems.
First, the pumping effect 170 of a flapping [plunging] airfoil 140
increases with increasing reduced frequency 150. The reduced
frequency k is defined as the product of the flapping [plunging]
foil's flapping frequency f times the flapping [plunging] foil's
chord length c divided by the flow velocity U, i.e., ##EQU2## A
flapping [plunging] foil 140 mounted in a boundary layer 10 always
operates in a very low velocity field 174 and therefore operates at
a high reduced frequency 145. This means that the flapping
[plunging] foil 140 generates a substantial jet effect 155 and
hence imparts a significant momentum increase 96 to the boundary
layer flow. This effect has been measured in the NPS water tunnel.
FIG. 5 shows the measured thrust 165 dependence on the reduced
frequency 145. It is seen that the thrust 165 increases
significantly with reduced frequency 145.
And Second, the pumping effect 170 of a flapping [plunging] airfoil
140 is enhanced if the flapping foil is mounted close to a wall 184
(as is necessarily the case if the flapping [plunging] airfoil 140
is located in a boundary layer 10). In recent tests in the NPS
water tunnel, quantitative measurements of the velocity
distribution 175 in the boundary layer 10 of a flat plate
(simulating a "wall" (184)) have been made. A small airfoil of 20
mm chord length was mounted next to a flat plate and oscillated in
the flapping [plunging] mode with non-dimensional flapping
amplitude of 0.088 at a reduced frequency of 57. FIG. 6 shows that
the jet velocity 155 downstream of the oscillating airfoil 140 that
is mounted next to the flat plate (wall) 184 is about twice as
large as the jet velocity 155 generated in the absence of the flat
plate or "wall". This favorable "wall effect" 185 has never been
demonstrated and measured prior to this experiment.
In aircraft engineering several attempts have been made to develop
"ornithopters". Such craft imitate bird flight and therefore are
based on the concept of flapping the complete wings. In contrast,
the proposed invention is based on the concept of flapping very
small foils in the boundary layer of a conventional fixed wing or
ship hull. Even though some people may be familiar with the fact
that flapping foils generate thrust, the proposed invention
requires the recognition that flapping foils generate a jet-like
velocity profile 155 downstream of the flapping [plunging] airfoil
140 which can be used for boundary layer re-energization and
propulsion 19. In contrast, a stationary (non-flapping) airfoil
generates a wake velocity profile 125, as shown in FIG. 3. The
recently completed NPS water tunnel tests are the first tests where
this jet-velocity 155 profile was measured (see FIG. 4) and where
it was established that this jet effect 155 is significantly
enhanced by the wall effect 185 (see FIG. 6). Furthermore, it
requires the recognition that it is advantageous to use boundary
layer propulsion 195 in spite of the fact that the installation and
operation of small vibrating foils on an aircraft wing or on a ship
hull is likely to be regarded as undesirable because of the
possible excitation of structural vibrations of the wing or
hull.
It is advantageous to provide a favorable pressure gradient 213,
see FIG. 2, on a lifting airfoil 5 as long as possible. Therefore,
the point of maximum thickness of the airfoil 5 should be located
as far downstream as possible, i.e., near the trailing edge of the
lifting airfoil. However, as can be seen on FIG. 2, the velocity
decrease and therefore the pressure increase may not exceed a
certain critical value if separation is to be avoided. This,
therefore, requires a certain distance between the point of maximum
thickness and the trailing edge. This distance can be shortened by
the use of an active boundary layer control, i.e.,
flapping/plunging foil 140. The installation of flapping foil
propulsors 140 in the concave part of the upper surface 7 of an
airfoil 5, as shown in FIG. 7, will capture the boundary layer 10,
prevent its separation, and thrust it toward the trailing edge of
the airfoil. The effectiveness of this arrangement has already been
demonstrated in NPS water tunnel tests. FIG. 8 shows an airfoil 5
with either a "cusped" 270 or with a rounded trailing edge 271. For
simplicity, the flapping [plunging] foil 140 was mounted in the
wake 125 downstream of the trailing edges. As expected, the flow
started to separate at the start of the adverse pressure gradient
region 215 (after the point of maximum airfoil thickness) if the
flapping [plunging] foil 140 was not oscillating. However, the
boundary layer flow separation 75 was suppressed for a wide range
of flapping frequencies and amplitudes as soon as the flapping foil
was excited into flapping motion. These results are summarized on
FIG. 9. It is seen that the effectiveness of suppressing the flow
separation increases with increasing amplitude of the flapping
foil.
In designing the flapping foil boundary layer propulsion system a
number of arrangements are possible. One such arrangement is shown
in FIG. 10 which presents the top view rather than the side view
shown in FIG. 7. The two flapping [plunging] foils 140 extend over
the full span of the conventional rectangular aircraft wing [air
foil 5]. They are held by two small support plates 141 at both
ends. The support plates 141 are excited into a sinusoidal flapping
motion by any suitable mechanical, electromechanical or hydraulic
excitation system. The manufacture and installation of such
excitation systems is state-of-the-art and therefore need not be
described in any specific detail. Furthermore, it is likely that
the flapping foils need to be supported and excited at several
locations along the span in order to avoid undesirable bending
modes and vibrations.
For ship propulsion, the arrangement shown in FIG. 11 will capture
the boundary layer 10 building up along a ship's hull 127. Here the
flapping [plunging] foils 140 are mounted near the ship's stern 128
and extend from the top to the bottom of the ship (not shown). As
before, the flapping foils 140 are attached to support plates 141
at both ends which are excited into flapping motion by any suitable
excitation system. Also shown are two flapping foils 140 mounted
close to but downstream of the ship's stern 128. These two foils
140 are mounted in close proximity to each other and are excited
into a flapping motion which occurs in counter-phase. This
arrangement is equivalent to a single flapping foil 140 near a wall
and producing the "wall" effect 185. Hence the jet enhancement
effect 155 will again be produced by this arrangement. Mounting
twin foil 140 propulsors in the wake 125 of a conventional ship,
similar to the arrangement shown in FIG. 11, will lead to the
suppression of boundary layer flow separation 75, similar to the
NPS test results shown in FIG. 9. The addition of a non-flapping
foil downstream from the hull will further enhance the achievable
thrust, due to the wave propeller effect discovered by Schmidt, or
it will produce a significant control moment if this additional
foil is deflected to some angle of attack.
Another arrangement is shown in FIG. 12. Here, a row of small
flapping foils 140 is mounted along the wing span similar to the
conventional vortex generators used on many aircraft. However, in
contrast to the rigidly attached vortex generators these small
flapping foils 140 are excited into a flapping [plunging] motion in
the spanwise direction (perpendicular to the flight direction).
These flapping foils 140 extend into the boundary layer 10 only
and, therefore, as in the previous arrangement, energize the
boundary layer 135. However, in this case, the vortices generated
by the flapping foils 140 are highly three-dimensional due to the
presence of tip vortices. Again, as before, any suitable excitation
system can be used for excitation of the flapping foils and
therefore no further detailed description of the various possible
excitation systems is provided.
Flow separation 75 may also occur in any duct flow 77 due to an
adverse pressure gradient 215. An arrangement suitable for duct
flow separation control is shown in FIG. 13. This arrangement is
similar to the one shown in FIG. 7. The flapping foils 140 span the
duct 76 and are excited into flapping [plunging] motion by any
suitable excitation system to reduce flow separation 35. For
example, flow separation 35 may occur in the diffuser 78 shown in
FIG. 13. A diffuser 78 is a duct 76 whose area widens in the
direction of the air/stream flow. Installation of flapping foils
140, as shown in FIG. 13 will minimize or suppress the boundary lay
flow separation 75.
In FIG. 14 another arrangement is shown which consists of an
ejector configuration 290 similar to the thrust augmenting ejectors
used on vertical take-off aircraft. However, instead of the primary
jet nozzle a flapping airfoil 140 is used to produce the primary
jet flow 291.
It will be known by those skilled in the art that various designs
of foil geometry for flapping [plunging] foil use on aircraft wings
or ship hulls are possible.
It will also be apparent to those skilled in the art that
alterations in duct flow geometry is possible, using various shapes
such as squares, rectangles, triangles, circles, ovals, ellipsoids,
and all other various shapes and combinations thereof.
It will be further apparent to those skilled in the art that the
specific mechanism by which the foils are energized into a flapping
motion, and other variations or modifications of the apparatus
and/or the methods of flapping the foils offer more possible
variations to this invention.
Obviously, many modifications and variations of the present
invention are possible in light of the above teachings. It is,
therefore, to be understood that the present invention may be
practiced within the scope of the following claims other than as
described herein.
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